The proposed studies are a continuation of ongoing research directed toward establishing a morphological substrate for synaptic associations between monoaminergic and peptidergic neurons in the brainstem, circumventricular organs, and basal ganglia of adult rats. The methods include dual light and electron microscopic immunocytochemical localization of neuronal antigens by the peroxidase-antiperoxidase (PAP) and immunogold techniques and combined PAP-localization of antigens and anterograde and retrograde transport. The antigens include the respective synthesizing enzymes for the catecholamines, Gamma-aminobutyeric acid (GABA), and acetylcholine; the beta 2-adrenergic receptor, and several neuropeptides (angiotensin, substance P, enkephalin, neurotensin, and neuropeptide Y). The objectives are outlined in four separate studies. Study I will examine the ultrastructure of adrenergic neurons and the synaptic interactions between GABAergic, catecholaminergic, and peptidergic neurons in the medial nuclei of the solitary tracts. Study II will examine the ultrastructural localization of angiotensin, the relations between catecholaminergic neurons and the beta receptor, and the synaptic interactions of GABA-ergic neurons in the subfornical organ and area postrema. Study III will examine the anatomical relations between dopaminergic afferents and intrinsic neurons or afferents containing various peptides, acetylcholine or the adrenergic receptors in the nucleus caudate-putamen. Study VI will examine the synaptic associations between dopaminergic neurons and other identified transmitters in the nucleus accumbes and the ventral tegmental area. Knowledge of the synaptic interactions between these neurotransmitter-specific neurons is important for establishing a neuroanatomical basis for normal functioning of the respective neurons in the animal model and possibly also in clinical manifestations such as hypertension and movement and emotional disorders involving the brainstem and basal ganglia.